(a) Field of the Invention
The present invention relates to liquid crystal displays (referred to as an LCD hereinafter) and manufacturing methods thereof, and more particularly, to circuits for preventing electrostatic discharge which are provided in the LCD and manufacturing methods thereof.
(b) Description of the Related Art
A liquid crystal display (LCD), which is one type of flat panel display (FPD), includes two substrates having transparent electrodes and a liquid crystal layer interposed between the substrates. In the LCD, light transmittance is controlled by varying the voltages applied to the liquid crystal layer.
On a thin film transistor (TFT) substrate of the LCD, N gate lines and M data lines, which cross each other, define a plurality of pixels In an N.times.M matrix. A pixel electrode is formed in each of the pixels and the pixel electrode is connected to the gate and the data lines through a switching device such as the TFT. The TFT controls display signals transmitted through the data line according to the states of the scanning signals transmitted through the gate line.
The majority of the LCD manufacturing process is performed on a glass substrate. Since the glass substrate is nonconductive, electric charges, which are abruptly generated on the substrate, cannot be dispersed. This may cause the insulating films or TFTs to become damaged by the electrostatic discharge.
In the LCD manufacturing process, since the electrostatic charges, which is generated after the step of assembling the TFT substrate and a color filter substrate is completed, cause high voltages even though the amount of the charges are small, the quality of the substrate decreases. In addition, since the electrostatic charge Is usually generated during the step of cutting the substrate, then flow into an active area having the pixel regions through gate and data pads, the channels of the TFTs near the pads become damaged by the electrostatic discharge with easy.
FIG. 1 shows a layout view of the conventional LCD substrate which is damaged by an electrostatic discharge. As shown in the drawing, the LCD panel includes a TFT substrate 10 and a color filter substrate 20. A pad area 30, in which pads are formed to connect each wire of the TFT substrate 10 to driving circuits, and an active area 40, in which actual images are displayed, are separately formed on the TFT substrate 10.
Lines 50 in the active area 40 illustrate pixels having some defects by damaged TFT portions. If electrostatic charges are generated in the pad area 30 and moves inside the active area 40, the channels of the TFTs, which are located near the pads, become damaged, or the quality of the channels is deteriorated.
The deteriorated TFT is shown in FIG. 2. As shown in the drawing, a gate line 60 and a data line 80 cross each other, and an edge of a gate electrode 61, extended from the gate line 60, overlaps an end of a source electrode 81 which is extended from the data line 80. An edge of the gate electrode 61, opposite the edge overlapping with the source electrode 81, overlaps with a drain electrode 82. A semiconductor film 70 is formed on the overlapping portion of the gate electrode 61, source electrode 81 and drain electrode 82.
If the electrostatic charges enter Into the TFT, comprised of the semiconductor film 70, the source and drain electrodes 81 and 82, and the gate electrode 61, sparks occur between the source and the drain electrodes 81 and 82, thereby damaging the semiconductor film 70.
To limit to the LCD by electrostatic discharge, a shorting bar, through which all metal wires are connected, is widely used for dispersing the electrostatic charges. However, in the case where an amount of the electric charges is large, it is not possible to completely prevent damage caused by the electrostatic discharge. Moreover, after the shorting bar is removed, it is not possible to prevent the electrostatic charges from entering into the substrate.
In manufacturing the LCD panel having the above structure, polarizers are attached after performing a visual display test by applying test signals to the shorting bar. Next, the main substrate is cut into individual LCD substrates, a liquid crystal material is injected between the substrates, and the injection holes are sealed. The shorting bar is removed in the step of cutting the substrate. In another visual display test, different test signals are applied to adjacent data lines by using probes directly contacted to each of the pads, then driving circuits are attached to the LCD panel.
As mentioned above, since the shorting bar is removed in the same step of cutting the substrate, it is difficult to protect the substrate against the electrostatic charges after the step of removing the shorting bar. Moreover, since the polarizers are attached after the simple test. In which only one signal is applied to every wire, by using the shorting bar, there is a high possibility that the polarizers are attached even on the damaged LCD panel. If the damaged panel is detected in the post-test, the panel, along with the expensive polarizers, has to be discarded, thereby increasing overall manufacturing costs of the LCD.